Electric resistance heaters are common place in industry, and generally comprise a resistance wire, through which an electric current is passed, a ceramic core, around which the same wire is disposed, a dielectric ceramic layer, which surrounds the current-carrying core, and a metal alloy sheath to complete the assembly. One form of electric resistance heater, known as a cartridge heater, which is used in a very wide range of applications, has a cylindrical sheath, which has historically been made of corrosion-resistant metal alloys such as stainless steel or incoloy. To enhance thermal performance of the heating element, the above assembly is typically swaged.
More recently, industry has been looking for alternative cartridge heaters that weigh less, cost less to produce, that can be designed with greater geometric flexibility, and that can be cost-effectively mass produced while yielding excellent thermal and mechanical performance. One solution was proposed in U.S. Pat. No. 5,586,214 to Eckman and jointly assigned to Energy Converters, Inc. of Dallas, Pa. and Rheem Mfg. Co. of New York, N.Y. Eckman discloses an immersion heater, somewhat similar to a cartridge heater in shape, but being hollow and having apertures in the sheath. Instead of being a solid cylinder, the core represents an injection molded polymeric hollow tube onto which a sheath is injection molded. Therefore, the heater does not have a "core" in the traditional sense. The Eckman heater is shown in FIG. 1.
The Eckman heater does have certain advantages over the prior art, such as low weight, low manufacturing cost at high volume, and its high resistance to galvanic corrosion and mineral depositing. Yet the Eckman heater has many limitations which leaves it undesirable for most applications other than low temperature and low heat flux water heating tanks.
This is supported by the limitation of thermoplastic matrices to accept filler medium. In this context, Eckman discloses that the filler level in these polymeric matrices cannot exceed 40% by weight, which correlates with the research results obtained during the development of the present invention.
Providing a solid core (or at least one of substantially greater wall thickness) in the Eckman heater is not as easy as changing the geometry of the polymer, around which the resistance wire is wound. If a core polymer with the same temperature dependent thermal expansion function as the outer polymer is used, the heater will be prone to cracking and failure when energized and brought to operating temperature. Eckman teaches that the outer polymer coating needs to be less than 0.5 inches and ideally less than 0.1 inches, which further sacrifices structural strength. Eckman achieves somewhat higher thermal conductivity and higher possible heat fluxes than would be found in a pure polymer by suggesting the use of carbon, graphite, and metal powder or flakes as an additive. The amount of these additives must be limited though to protect the heater's dielectric strength. Even then, thermal conductivity does not get significantly better than 1.0 W/(m*K).
It is also preferable to have other components of thermal systems, such as sensors, control systems, and thermoelectric modules that can be used for either cooling or heating purposes, placed in a low weight, low cost housing. Most of these components however are not suitable or only marginally suitable for molding within an injection molded polymer. This is due, in part, to the high temperatures to which the parts are exposed during molding, and also in part due to the marginal rewetting capabilities of many injection molded plastics. Even with components that are not as vulnerable to the high temperatures of injection molding, such as some temperature sensors, the low thermal conductivity of these polymers, as mentioned above, limit the usefulness of the finished product.
It is thus an object of the present invention to provide a molded polymer composite heater with a composite filler level of substantially greater than 40%.
It is also an object of the present invention to provide a molded polymer composite heater with improved structural integrity.
It is further an object of the present invention to provide a molded polymer composite heater with greater core thickness up to the extreme where the hollow space in the center of the element vanishes.
It is yet another object of the present invention to provide a molded polymer composite heater with improved thermal performance, namely thermal conductivity and maximum heat flux.
It is still a further object of the present invention to provide a variety of electrical components for thermal systems encased in polymer sheaths.
Other objects of the invention will become apparent from the specification described herein below.